Tiki made a little belt similar to the back support belts with two pockets over your kidneys. It had 2 heating pads like the hand warmers you can get that you boil in water then crack off and they give off the heat. I never tried one but I reckon they'd be nice.
Would take the edge off on a cold day and stop your lower back tightening up.

The whole concept of those devices is that through the kidneys circulate a lot of blood so by warming up the kidneys you warm up the blood that will be circulating through your body hence producing the warming sensation.

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Shoot it would be great to have some warming options for those of us who have injuries. I wish I had some constant heat wrapped around my knee. I used to have one of those boil packets that you crack before use. The thing was awesome, a pain to recharge since you had to boil it every time and it only lasted an hour or so but it got hot for sure. 130 degrees if I remember the specs correctly.

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As far as those toe warmer type things, those will not work in water. It there was a good way to heat the toes in the water, it might work as well as kidney packs. The idea being that your heart stops pumping warm blood to your extremities where there is no warmth to prevent the colder blood coming back up. If you can get those areas warm, your body will then again pump warm blood down there and in turn raise your core temp a little as well. I'm not sure how long those vest last but I know skiboot warmers can hold up for several hours depending on the heat setting. It might be possible to rig those batteries and components up in some watertight baggies and attach them to your footstraps.